1. Field of the Invention
The present invention relates to improved methods, compositions and fluid loss control additives for cementing subterranean zones penetrated by well bores.
2. Description of the Prior Art
Subterranean zones penetrated by well bores are commonly sealed by hydraulic cement compositions. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annual sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and seals the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in a variety of cementing operations such as sealing highly permeable zones or fractures in subterranean zones, plugging cracks or holes in pipe strings and the like.
Fluid loss control additives have been included in hydraulic cement compositions to prevent significant loss of fluid, i.e. water, into the subterranean zone or formation being cemented. In primary cementing, the loss of fluid from the cement composition can result in premature gelation of the cement slurry or bridging of the annular space between the formation and the casing.
While a variety of fluid loss control additives have been utilized in hydraulic cement slurries used in cementing subterranean zones penetrated by well bores, they are not environmentally acceptable, especially in off shore operations in that they do not undergo complete biodegration in the environment and cause damage to aquatic and other life therein. Thus, there are continuing needs for improved biodegradable cement composition fluid loss control additives and improved cement compositions containing such additives.